1. Field of the Invention
The present invention relates to an optical transmitter in an optical communication system such as a wavelength division multiplexing (WDM) communication system.
2. Description of the Related Art
In a high speed optical fiber communication system, waveform distortion is caused by a synergistic effect between wavelength dispersion of a transmission line formed by optical fibers and non-linear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM), so that the transmission speed characteristics would be degraded and the transmission distance would be decreased.
A first prior art optical transmitter using a conventional return-to-zero (RZ) format is constructed by a clock modulator formed by a single-end type intensity modulator for performing a clock modulation upon a carrier optical signal using a clock signal having a frequency of a bit rate of an NRZ data signal, and a data modulator for performing a data modulation upon the optical output signal of the clock modulator using the NRZ data signal. This will be explained later in detail.
In the above-described first prior art optical transmitter, however, since the spectrum bandwidth of a main lobe is relatively large, the optical spectrum of neighboring channels may be superposed onto each other in WDM systems, so that the neighboring channels easily interfere with each other. Also, to reduce the waveform distortion caused by a synergistic effect between wavelength dispersion of the optical fiber and nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM), the transmitted power is preferably relatively small, which decreases the transmission distance.
A second prior art optical transmitter using a carrier-suppressed return-to-zero (CS-RZ) format includes a clock modulator formed by a push-pull type intensity modulator (see: Y. Miyamoto et al., “320 Gbit/s (8×40 Gbit/s) WDM transmission over 367 km with 120 km repeater spacing using carrier-suppressed return-to-zero format”, Electronics Letters, Vol. 35, No, 23, pp. 2041-2042, Nov. 11, 1999). This also will be explained later in detail.
In the above-described second prior art optical transmitter, since the spectrum bandwidth of a main lobe is relatively small, the optical spectrum of neighboring channels may be hardly superposed onto each other in WDM systems, so that the neighboring channels hardly interfere with each other. Also, since the waveform distortion caused by a synergistic effect between wavelength dispersion of the optical fiber and non-linear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) is suppressed, the transmitted power can be relatively large, which upgrades the transmission speed characteristics and increases the transmission distance.
In the above-described second prior art optical transmitter, however, the adjustment of phase and amplitude between the clock signals and the adjustment of a bias voltage are very troublesome.
A third prior art optical transmitter using an alternate-chirped return-to-zero (AC-RZ) format includes a clock modulator formed by a single-end type phase modulator and an optical filter (see: R. Ohhira et al. , “Novel RZ signal format with alternate-chirp for suppression of nonlinear degradation in 40 Gb/s based WDM”, OFC2001, WH2-1˜WH2-3, March 2001). This also will be explained later in detail.
Even in the above-described third prior art optical transmitter, since the spectrum bandwidth of a main lobe is relatively small, the optical spectrum of neighboring channels may be hardly superposed onto each other in WDM systems, so that the neighboring channels hardly interfere with each other. Also, since the waveform distortion caused by a synergistic effect between wavelength dispersion of the optical fiber and non-linear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) is further suppressed, the transmitted power can be relatively large compared with the above-described second prior art optical transmitter, which further upgrades the transmission speed characteristics and further increases the transmission distance. Further, the adjustment of phase and amplitude of the clock signal and the adjustment of a bias voltage are very easy.
In the above-described third prior art optical transmitter, however, the clock modulator and the data modulator cannot be integrated into one substrate made of LiNbO3 or LiTaO3, because the optical filter cannot be formed on such a substrate.